Delving deep into the structural aspects of a furin cleavage site inserted into the spike protein of SARS-CoV-2: A structural biophysical perspective

One notable feature of the SARS-CoV-2 genome, the spike (S) protein of SARS-CoV-2 has a polybasic furin cleavage site (FCS) at its S1-S2 boundary through the insertion of 12 nucleotides encoding four amino acid residues PRRA. Quite intriguingly, this polybasic FCS is absent in coronaviruses of the same clade as SARS-CoV-2. Thus, with currently available experimental structural data for S protein, this short article presents a set of comprehensive structural characterization of the insertion of FCS into S protein, and argues against a hypothesis of the origin of SARS-CoV-2 from purposeful manipulation: (1), the inserted FCS is spatially located at a random coil loop region, mostly distantly solvent-exposed (instead of deeply buried), with no structural proximity to the other part of the S protein; (2), the insertion of FCS itself does not alter, neither stabilize nor de-stabilize, the three-dimensional structure of S; (3), the net result here is the insertion of a furin cleavage site into S protein, whose S1 and S2 subunits will still be strongly electrostatically bonded together from a structural and biophysical point of view, even if the polybasic FCS is actually cleaved by furin protease before or after viral cell entry.